Having a free spine is always something I think of when practicing. If you walk away from yoga class with a sore back then there is something about your practice that is not right. It should not hurt to do yoga.
One of the most common actions I perform in my practice is to lengthen the lower back. I do this by imagining that my sitting bones are melting down the back of my legs. I like the idea and feeling of melting as it reminds me this is something done without force. I once told a class to imagine their butt cheeks were like two scoops of ice cream atop a cone (their legs) and that the ice cream was dripping down the one (i.e., their legs). A child, who happened to be in the class with her mother, whispered loudly, "My bottom is not an ice cream.' Some visualisations are not for everyone I guess!
Anyway, the idea is to create some space in the lower back. It is a small action done without strain or force. If you overdo it, or misunderstand the movement as something more akin to what I call a pelvic tuck, then you might irritate nerves if you have an irritable nerve condition so be mindful and move slowly.
I try to demonstrate the difference between the lengthening movement (melting) and the tuck in the video. What you will notice is that no length is created in the lower back with just the tuck (the way I perform it in the video). Whereas, when I melt the sitting bones down you will see that the space between my vertebrae is lengthened.
I use this melting of the sitting bones to prepare for standing poses, in my forward bends, and in my back bends. It helps to prevent me from overarching the back or squashing it. If you combine it with lifting the shoulders and arms overhead it will traction the whole spine.
These videos are intended mainly for students who attend my classes as an aid to their practice and as a point of discussion. Please be mindful if you are practicing any videos from any source without the guidance of an experienced teacher.
Happy and safe practicing!